Audiophile/physicist advances 3D audio

A new 3D audio technology using a crosstalk cancellation technique works with stereo loudspeakers and can be used to enhance existing stereo recordings.

A new 3D audio technology has been in the news recently. What's notable about this latest approach is that it works with stereo loudspeakers - not headphones - for proper playback and can be used to enhance existing stereo recordings.

The technique, developed by Edgar Choueiri, a professor of applied physics and aerospace engineering at Princeton University, uses optimized crosstalk cancellation filters to allow binaural cues in a recorded signal to reach a listener's ears with minimal degradation. During normal (non-filtered) playback of binaural sound through loudspeakers, these spatial cues are significantly degraded as a result of the crosstalk that exists between the respective loudspeakers and listener's ears.

This approach appears to offer advantages over other loudspeaker-based 3D audio techniques (such as ambisonics and wavefield synthesis) as it requires only two loudspeakers for playback and is compatible with existing stereo recordings. In fact Choueiri claims that crosstalk cancellation will "significantly benefit the realism" of such recordings, since most contain significant binaural cues.

Of course, there's no such thing as a free lunch and most audiophiles will no doubt cringe at the thought of adding extra circuitry - especially crosstalk cancellation filters, which have a reputation for adding significant spectral coloration to the sound - into their audio playback system signal path. However, Choueiri, an audiophile himself, claims that his processing technique was developed with that concern in mind and that it adds nothing artificial to the sound.

Implementation in an end-user audio system is achieved through a digital processor unit that applies - in real time - a customized digital filter to any stereo sound input. The filter is created using on-site, in-position acoustic measurements of the entire listening chain and recordings made from miniature microphones placed at the entrance of the listener's ear canals.

This technique isn't limited to consumer in-home audio system use. Other applications mentioned include 3D audio for 3D TV and 3D cinema, as well as advanced hearing aids.

An interesting short (6:44) video - "Introduction to 3D Audio with Professor Choueiri" - is available on the home page of Choueiri's 3-D Audio and Applied Acoustics Laboratory web site. The video includes animations demonstrating the 3D audio technique, as well as some actual 3D audio demos. (Check out the buzzing fly!)

If you want true spatial 3D sound, all elements of the sound or recording should be individually spatialized. That is best accomplished using a monaural sound which is then positioned in 3D space using Head Related Transfer Function calculations and then applying crosstalk cancellation to the 2 speaker configuration. The result should approximate what each ear should hear (binaural hearing). The best result would be in headphones which can provide the most accurate binaural representation of the spatialized sound.

I skimmed through the paper and it does look like pretty decent stuff, as well as being reasonably well-documented (Cooper-Bauck cited early on among others; Bauck is misspelled but so it goes). And I realize that the article doesn't exactly say that other virtualization techniques require greater-than-2 loudspeakers, but it could give that impression. Certainly most wavefield synthesis requires a whole bunch.
Chouieri is also candid about various pitfalls in his paper, including the fact that many pop recordings are done in ways that will map ineffectively to his approach.
The Bach tribute at the end is cute.

Crosstalk cancellation goes way back, all the way to Blumlein, and there are many, many more recent patents (Mouri, Cooper-Bauck, etc.). I'm sure the good Professor has done some refinements focusing on specific binaural aspects, and I will have to check them out --- but the article's implication that "other loudspeaker-based 3D audio techniques" require more than two loudspeakers is quite incorrect.